This laboratory will continue to investigate the reaction sequence of the sodium and potassium ion pump through the kinetics of phosphorylation and dephosphorylation of the Na ion, K ion-ATPase in membranes derived from homogenates of guinea pig kidney. Phosphorylation will be studied both from (P32)ATP and radioactive inorganic phosphate. Dephosphorylation will be observed by chasing the phosphorylated enzyme with unlabeled substrate or by an excess of a chelator of Mg2 ion, which is required for phosphorylation. The following problems will be investigated. 1) Competition between Na ion and K ion for control of active centers for phosphorylation and dephosphorylation. 2) The rapid kinetics of these reactions will be studied with a rapid mixing apparatus. 3) The nature of the active site labeled with radioactive 8-azido ATP in the presence of ultraviolet light. Bibliographic references: Post, R.L., Toda, G., and Rogers, F.N. (1975) J. Biol. Chem. 250, 691-701. Phosphorylation by Inorganic Phosphate of Sodium Plus Potassium Ion Transport Adenosine Triphosphatase. Four Reactive States. Taniguchi, K. and Post, R.L. (1975) J. Biol. Chem. 250, 3010-3018. Synthesis of Adenosine Triphosphate and Exchange between Inorganic Phosphate and Adenosine Triphosphate in Sodium and Potassium Ion Transport Adenosine Triphosphatase.